Speckled dino spurs debate about ancient animals’ colors | Science News for Students

Speckled dino spurs debate about ancient animals’ colors

Fossilized pigment pouches hint at ancient animals’ lives — but are open to interpretation
Nov 29, 2016 — 7:00 am EST
dino camo

The plant-eating Psittacosaurus may have had a dark back and light belly, shown in this artist’s creation. The coloring could have acted as camouflage, helping the dinosaur hide in forested habitats.

J. Vinther/University of Bristol, Bob Nicholls/Paleocreations.com

Picture a dinosaur about the size of a golden retriever. Make it walk on two legs. Give it freckles. Then give it a dark back and a light belly. That’s a pretty good picture of one plant-eating dinosaur.

Named Psittacosaurus (Sit-TAK-uh-SOR-us), “It would have been a super-cute animal,” says Jakob Vinther. “It’s got this wide face and looks a little bit like E.T.”

A paleontologist, Vinther studies fossilized animals and plants. Over the past few years, he has become increasingly interested in the color of dinosaurs. So have other scientists. Now, there is a debate going on about just how much we know about what dinosaurs looked like and what their colors tell us about their lives.

The debate stems from the way scientists interpret certain structures that have been preserved in dinosaur fossils. Vinther and others say these structures could be the leftovers of ancient pigments. They are referring to pigment structures known as melanosomes (Meh-LAAN-oh-soams). These give color to skin, feathers and other tissues. They also might linger in fossil skin.

Finding ancient pigments in dinosaur fossils could open up a wide new world of dinosaur biology. For instance, it might answer all sorts of lifestyle questions, says Hannah Rowland. She is a zoologist, which means she studies animals, at the University of Cambridge, in England. “You might be able to take a fossil … and infer a dinosaur’s life history just from its pigment patterns,” she says. “That’s the most exciting thing.”

Not so fast, says Mary Schweitzer. She is a paleontologist at North Carolina State University in Raleigh. Evidence for ancient pigments can be hard to interpret. In some cases, microscopic structures that appear to be melanosomes may actually be microbes, she says. “Both hypotheses,” she says, “remain viable until one is shot down with data.” So for now, she argues, inferring dinosaur lifestyles from alleged ancient pigments is impossible.

From color to camouflage

That hasn’t stopped Vinther and others from trying. Psittacosaurus is the latest example.

The fossil is spectacular, Schweitzer says. “It’s got skin all over the place. I can’t think of too many dinosaur specimens that are preserved like this.”

Psittacosaurus fossil
This is a complete skeleton of Psittacosaurus, including fossilized skin. Black speckles cover the skin and could offer clues to the color of the dinosaur.
J. Vinther/University of Bristol, Bob Nicholls/Paleocreations.com

The fossil dinosaur lies on its back. It was flattened in a slab of volcanic rock. Skin covers a completely intact skeleton. Dozens of long bristles poke from its tail. The dino ate plants and lived some 120 million years ago. Walking on two legs, it would have stood some half meter (roughly 20 inches) tall.

Black material speckles the dinosaur’s body, tail and face. Vinther believes the material is the ancient remains of pigment — those melanosomes. His team examined samples chipped from the fossil. The researchers saw what seems to them the telltale orbs of melanosomes. The signs were mostly impressions in the rock. But there were some true 3-D structures too.

These structures are part of the reason the team believes the dinosaur was colored. It would have had a dark back that faded to a lighter belly, they say. That type of coloring is called countershading. From penguins to fish, it can serve as a type of camouflage. It lightens parts of the body typically in shadow. It darkens those parts typically exposed to light. “If you want to hide,” Rowland says, “it makes sense to try and obliterate those shadows.”

Vinther’s team also wanted to figure out where Psittacosaurus may have lived. So they crafted two life-size models of the dinosaur. One was painted with the color patterns inferred from the fossil. The other was made a uniform gray. The researchers then photographed the gray model in diffuse light. This is the type that would filter down through a forest. They photographed their models in direct light too. This is the type of direct sunlight that occurs on a prairie or a savanna. 

Different lighting conditions made different shadow patterns on the model dinosaur’s body.

The team then figured out how to best disguise these patterns in the two different lighting scenarios. Their disguise for diffuse light matched the model painted like Psittacosaurus. “It’s like what we see in forest-living animals,” Vinther says. That's why they now conclude: “This thing was camouflaged.”

Vinther published his team’s work on Psittacosaurus in the September 26 Current Biology. Their study, while exciting, has exposed pitfalls in research on paleo color. That is the study of fossil pigments and what they can reveal about ancient animals.

Still, the promise is clear: Paleo color could paint a vivid picture of a dinosaur’s life. It could offer clues about dinosaur behavior, habitat and evolution. “This is a crucial new piece in the puzzle of how the past looked,” Vinther claims.

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mesh form dino

clay dino model

dino cast

A field emerges

Paleontologists have been puzzling for centuries over what ancient animals might have looked like. Then, eight years ago, these scientists got a wake-up call. That’s when Vinther and his colleagues made a stunning suggestion. They had been looking at microscopic structures in a roughly 125-million-year-old fossil feather. The tiny structures, these researchers argued, were actually a type of melanosome. They claimed to see pigment pouches that rest inside pigment cells. In this particular fossil, the pigments might have given the feather a blackbird’s raven-like hue.

Scientists had noticed similar structures inside fossilized skin and feathers before. They had spied them since the early 1980s. But those scientists had assumed these structures were remnants of bacteria. Perhaps they were the remains of microbes that had feasted on the animal's carcass, says Martin Sander. He is a paleontologist at the University of Bonn in Germany.

Then the new way of looking at things by Vinther’s team sparked a flurry of additional research. Scientists have since spotted what appear to be melanosomes in all types of fossilized animals. Paleontology, in fact, is now full of colors and patterns. For instance, pigment pods may have painted reddish-brown speckles upon the face of a dinosaur called Anchiornis huxleyi (AN-kee-OR-nis HUX-lee-eye).

Pigments appear to have painted chestnut stripes on a long-tailed dino from China. And they made the feathers iridescent on a four-winged dinosaur called Microraptor. (Iridescence is when an object seems to change color with a shift in the angle at which it is seen.) The shimmery dinosaur “probably had a weak, glossy iridescence all over its body,” concludes Matthew Shawkey. He is an evolutionary biologist at Ghent University in Belgium. His team deduced Microraptor’s color from the shape of its melanosomes.

Linking colors and shapes

Modern melanosomes generally carry a mixture of two pigments. Each is a type of melanin (MEL-uh-nin). A dark brown-black type is known as eumelanin (YOU-mel-uh-nin). The other, red-yellow version is called pheomelanin (FEE-oh-MEL-uh-nin). Scientists have also linked the colors in mammals and birds to a melanosome’s shape. A meatball shape points to reddish-brown hues. A sausage shape tends to provide darker colors.

In feathers that shimmer, melanosomes tend to be even thinner. Microraptor’s melanosomes looked like skinny sausages. They were similar to those seen in the feathers of modern crows and ravens, says Shawkey. He, Vinther and their team published these observations in Science four years ago.

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Melanosome shapes
Melanosomes come in different shapes and carry different pigments. These shapes have been seen in fossilized and living animals. Pigments may be mixed in melanosomes. Iridescent ones (shown as purple) are arranged in orderly structures.
T. TIBBITTS; Source: Q. Li et al/Science 2012

Last year, Vinther laid out the case for inferring color — and ancient histories —from fossilized pigments. He published his ideas in a review in Bioessays. The distinctive shapes of melanosomes offer clues to color, he claimed. Chemical tests could also help detect the presence of melanin itself. To find this pigment in fossils, he argued, put the old bacteria hypothesis to bed.

Schweitzer’s group disagreed. They wrote a review published in Bioessays later in the year. Researchers need to be cautious, these scientists countered. Deducing the hues of extinct animals isn’t easy. Any melanosome look-alikes in fossil feathers or skin might actually be microbes, they argued. After all, microbes are everywhere.

“These animals died in an environment that was not sterile,” Schweitzer points out. “Think about it. If you take a piece of chicken and throw it out in your backyard, how long does it take for microbes to overgrow that chicken?”

Microbes are hardy, too. They and the sticky biofilms that they create can be preserved in fossils. Schweitzer also notes that microbes and melanosomes overlap completely in shape and size. That can make the two hard to tell apart. What’s more, some microbes actually make melanin. So finding the pigment in a fossil is not proof that the ancient critter was black, brown or freckled.

snake skin
Degraded snake skin contains a mix of microstructures. In a scanning electron microscope image (left), bacteria and melanosomes are hard to tell apart. But in the same tissue, observed under a transmission electron microscope (right), melanosomes appear as black spots. Bacteria, in contrast, appear translucent.
J. LINDGREN/LUND UNIV.

Johan Lindgren coauthored the Bioessays article with Schweitzer. A geologist at Lund University in Sweden, he has no doubt that melanosomes can leave traces in the fossil record. The issue, Lindgren says, is that not every round structure will be a melanosome.

Chemical tests could help distinguish one from the other. Bacteria, for instance, leave behind telltale traces. These can be identified with a specific type of chemical test. It uses high heat to break molecules down into smaller bits. A machine then determines the chemical composition of those bits.

But using this technique requires burning up the sample. “It can mean destroying much of what you are trying to study,” points out Roy Wogelius. He is a geologist at the University of Manchester in England. So this type of testing “is not always possible.”

Vinther’s new work is not likely to settle the debate. In fact, people argued both sides of the issue in October at a meeting in Salt Lake City of the Society of Vertebrate Paleontology.

Arindam Roy is a colleague of Vinther’s at Bristol. At that meeting, he reported size differences between fossilized melanosomes and the bacteria growing on decaying chicken feathers in the lab.

Alison Moyer said those observations weren’t enough, though. She is a colleague of Schweitzer’s at North Carolina State. She says keratin is key. Keratin is a protein that typically surrounds melanosomes. It, Moyer says, could serve as evidence for pigments in fossils.

Lingering doubts

Psittacosaurus’ skin very well may contain ancient pigments, Wogelius says. “I don’t think it’s a crazy idea.” However, he says of Vinther’s group: “I don’t think they’ve proved what they claim.”

Vinther’s team, for example, used just four tiny fossil samples to extrapolate the coloring of a whole dinosaur. “I think it’s a bit of an overreach,” Wogelius says.

Schweitzer also notes that the specimen had been varnished. That means it probably had been painted with some type of gloss. This likely was done to protect the fossil. It happened before Vinther and colleagues got their hands on the dinosaur. That gloss now makes it impossible to perform the chemical tests that would bolster any claim for pigments.

Psittacosaurus
Only about the size of a golden retriever, Psittacosaurus had a distinct head, which gave it a bit of the look of E.T., the Extra-Terrestrial (from the 1982 movie).
J. Vinther/University of Bristol, Bob Nicholls/Paleocreations.com

“Varnish is horribly destructive to fossils,” Schweitzer says. “It totally ruins the specimen for other types of analysis.”

Vinther argues that his team has chemically analyzed other fossils and found evidence of melanin — not bacteria. And, he adds, the structures in those fossils look just like the ones in Psittacosaurus.

Vinther’s team also saw evidence of just one kind of micro-structure. It had a quite round shape. If these structures were actually bacteria, he says, you’d expect to see a whole range of shapes and sizes. “Some of them would be shaped like corkscrews. Some would have flagella. Some would be humongous. Some would be tiny.”

Not necessarily, counters Lindgren. That’s the tricky thing with bacteria. “In some cases you can have a huge consortium," she says, meaning lots of varied types. "In other cases you can have one single type.”

But you don’t have to rely on that argument alone, a new study finds. It has just turned up melanosomes and beta keratin — a protein in the stringy matrix that surrounds melanosomes. “Identifying keratin is key to ruling out a microbial source,” write Mary Schweitzer and her colleagues.

This newest report appeared online November 21 in the Proceedings of the National Academy of Sciences.

Schweitzer’s team had been examining feathers from a 130-million-year-old bird known as Eoconfuciusornis (EE-oh-kon-few-shious-SOR-nis). And this group, too, found round, 3-D structures. 

The authors won’t speculate on the ancient bird’s color. They do, however, argue that the beta keratin offers a new way to support claims of finding ancient pigments.

More research hoped for

Vinther’s argument that he’s found melanosomes certainly has its backers. “I was skeptical at first,” says the University of Bonn’s Sander. “But now there’s been such an array of these little [structures] that it’s pretty clear that at least some of them are not bacteria.” Despite some continuing controversy, he says many paleontologists now accept that some fossil micro­structures may be melanosomes.

More research, though, would help remove “any lingering doubts,” he says.

Along with chemical tests, Schweitzer suggests, researchers could try transmission electron microscopy (TEM). This technique blasts an electron beam through a thinly sliced sample. With TEM, melanosomes appear as black blobs. Bacteria tend to look quite different — in some cases, like fried eggs.

Ghent University’s Shawkey is instead focusing on chemistry. He published a paper online November 14 in Palaeontology. In it, his team used a technique called Raman spectroscopy. This method helps produce a chemical fingerprint of a sample.

These researchers used the tool to help build a case for feather coloring. They were looking at feathers from a bird that died some 120 million years ago. The researchers spotted skinny sausages in the feathers. These are a tell-tale sign of iridescent melanosomes. There also was evidence of the pigment eumelanin. 

No doubt, the field of paleo color has its snags. But the emerging work may mean paleontologists can begin to fill in the hues and tints of ancient animals. And that might one day reveal the habits and habitats of creatures that, until recently, had been known almost exclusively by their bones and teeth.

Power Words

(more about Power Words)

behavior     The way a person or other organism acts towards others, or conducts itself.

biofilm     A gooey community of different types of microbes that essentially glues itself to some solid surface. Living in a biofilm is one way microbes protect themselves from stressful agents (such as poisons) in their environment.

biology     The study of living things. The scientists who study them are known as biologists.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

camouflage    Hiding people or objects from an enemy by making them appear to be part of the natural surroundings. Animals can also use camouflage patterns on their skin, hide or fur to hide from predators.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.

chromatography     Separation and detection of chemical compounds as a result of their having traveled at different rates according to their different attractions to the matter that carries them (such as a flowing liquid or gas).

colleague     Someone who works with another; a co-worker or team member.

consortium     A group or association of independent organizations.

crow     The characteristic loud cry of a rooster.

current     A fluid body — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

dinosaur   A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary biologist     Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).

extinct     An adjective that describes a species for which there are no living members.

extrapolate     To estimate a number based on some existing data, when those data are aren’t sufficient to be able to calculate a number directly.

flagella     (sing. flagellum)   A thread-like structure that comes out of certain types of cells. The term is derived from the Latin word for whip. And that's because these structures serve like oars to help the cells travel. 

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. 

forest     An area of land covered mostly with trees and other woody plants.

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

hypothesis     A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

infer    (n. inference) To conclude or make some deduction based on evidence, data, observations or similar situations.

iridescent     Adjective that describes something that seems to change color with a shift in the angle at which it is viewed or at which lighting is applied.

keratin     A protein that makes up your hair, nails and skin.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

mass spectrometry     A technique used to determine the chemical makeup of a source material.

melanin    A family of pigments found in all types of animals. They are responsible for the dark coloring in such things as feathers, hair, fur, skin and scales.

melanosome  A structure within a cell that gives an organism color.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view such tiny objects, such as bacteria or other one-celled organisms.

model     A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

online     A term that refers to things that can be found or done on the Internet.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

paleontology  The branch of science concerned with ancient, fossilized animals and plants. The scientists who study them are known as paleontologists.

penguin     flightless black-and-white bird native to the far Southern Hemisphere, especially Antarctica and its nearby islands.

pigment     A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

prairie     A type of fairly flat and temperate North American ecosystem characterized by tall grasses, fertile soils and few trees.

protein     Compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins. Medicines frequently work by latching onto proteins.

pyrolysis gas chromatography-mass spectrometry   In this technique, high heat breaks down molecules into smaller bits. A machine then determines the chemical composition of those bits. It usually requires the sample be vaporized.

Raman spectroscopy    A chemical tool that helps provide a chemical fingerprint of some sample.

remnant     Something that is leftover — from another piece of something, from another time or even some features from an earlier species.

salt     A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine .

savanna     A grassland sometimes also populated with trees. Most are fairly dry for part or much of the year.

scenario     An imagined situation of how events or conditions might play out.

skeptical     Not easily convinced; having doubts or reservations.

sterile     An adjective that means devoid of life — or at least of germs. (in biology) An organism that is physically unable to reproduce.

tissue     Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

translucent       The property of letting light through, but not being transparent. Usually, things viewed through a translucent material (such as frosted window glass) appear as hazy shapes with no detail.

transmission electron microscopy (or TEM)    This technique blasts an electron beam through a thinly sliced sample. An image is formed from the interaction of the electrons transmitted through the specimen.

vaporize    To convert from a liquid to a gas (or vapor) through the application of heat.

vertebrate     The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.

NGSS: 

  • MS-LS1-1
  • MS-LS1-5
  • MS-LS4-1
  • MS-LS4-4
  • HS-LS4-1
  • HS-ETS1-2

Citation

Journal: J. Vinther et al. 3D camouflage in an ornithischian dinosaur. Current Biology. September 26, 2016. doi:10.1016/j.cub.2016.06.065.

Journal: J. Vinther. A guide to the field of palaeo color. Bioessays. Vol. 37, June 2015, p. 643. doi:10.1002/bies.201500018.

Journal: M.H. Schweitzer, J. Lindgren and A. E. Moyer. Melanosomes and ancient coloration re-examined: A response to Vinther 2015. Bioessays. Vol. 37, November 2015, p. 1174.  doi: 10.1002/bies.201500061.

Journal: Q. Li et al. Reconstruction of Microraptor and the evolution of iridescent plumage. Science. Vol. 335, March 9, 2012, p. 1215. doi: 10.1126/science.1213780

Further Reading